84289

ING5

p28ING5;inhibitor of growth family, member 5;ING5;inhibitor of growth family, member 5

inhibitor of growth protein 5

2q37.3

protein-coding

Members of the ING family of tumor suppressors regulate cell cycle progression, apoptosis, and DNA repair as important cofactors of p53. ING1 and ING3 are stable components of the mSin3A HDAC and Tip60/NuA4 HAT complexes, respectively. We now report the purification of the three remaining human ING proteins. While ING2 is in an HDAC complex similar to ING1, ING4 associates with the HBO1 HAT required for normal progression through S phase and the majority of histone H4 acetylation in vivo. ING5 fractionates with two distinct complexes containing HBO1 or nucleosomal H3-specific MOZ/MORF HATs. These ING5 HAT complexes interact with the MCM helicase and are essential for DNA replication to occur during S phase. Our data also indicate that ING subunits are crucial for acetylation of chromatin substrates. Since INGs, HBO1, and MOZ/MORF contribute to oncogenic transformation, the multisubunit assemblies characterized here underscore the critical role of epigenetic regulation in cancer development.

Our previous study showed high frequency of allelic loss at chromosome 2q37 region in oral cancer. This location contains several candidate tumor suppressor genes such as PPP1R7, ILKAP, DTYMK and ING5. We previously showed 3 members of inhibitor of growth (ING) family, ING1, ING3 and ING4 as tumor suppressor gene in head and neck cancer. As ING5 shows high homology with other members of ING genes including highly conserved carboxy-terminal plant homeodomain and nuclear localization signal, we first picked up ING5 and examined it as a possible tumor suppressor in oral cancer. For this aim, mutation and mRNA expression status of ING5 in paired normal and oral squamous cell carcinoma samples were examined by reverse transcription polymerase chain reaction (RT-PCR) and sequencing. Three missense mutations located within leucine zipper like (LZL) finger and novel conserved region (NCR) domains in ING5 protein were detected, probably abrogating its normal function. We also found 5 different alternative splicing variants of ING5. Then, we examined mRNA level of ING5 by quantitative real time reverse transcription polymerase chain reaction (qRT-PCR) analysis, which demonstrated decreased expression of ING5 mRNA in 61% of the primary tumors as compared to the matched normal samples. In conclusion, tumor-specific mutation and downregulation of ING5 mRNA suggested it as a tumor suppressor gene in oral squamous cell carcinoma.

PURPOSE: This study aimed to assess the protein level of inhibitor of growth gene 5 (ING5) in head and neck squamous cell carcinoma (HNSCC) and to explore its roles in tumorigenesis and cancer progression. METHODS: ING5 expression was assessed in 172 cases of HNSCC by immunohistochemistry using tissue microarray, and in 3 oral SCC cell lines by immunohistochemistry and Western blot. expression of ING5 was compared with clinicopathological variables, TUNEL assay staining, and the expression of several tumorigenic markers. In addition, double immunofluorescence labeling was performed in order to analyze the colocalization of ING5 with p300 and p21. RESULTS: ING5 expression was primarily observed in the nuclei, but was also occasionally found in the cytoplasm of both SCC cell lines and tissue samples of HNSCC. Nuclear expression of ING5 in HNSCC was significantly lower than that of non-cancerous epithelium, and was positively correlated with a well-differentiated status. In contrast, cytoplasmic expression of ING5 was significantly increased in HNSCC, and was inversely correlated with a well-differentiated status and nuclear ING5 expression. In addition, nuclear expression of ING5 was positively correlated with p21 and p300 expression, and with the apoptotic index. In contrast, cytoplasmic expression of ING5 was negatively correlated with the expression of p300, p21, and PCNA. Although no statistical association was found between the expression of nuclear ING5 and mutant p53 in HNSCC, patients with high expression of nuclear ING5 tended to have converse prognoses when grouped according to mutant p53 expression. CONCLUSIONS: Our results suggest that a decrease in nuclear ING5 localization and cytoplasmic translocation are involved in tumorigenesis and tumor differentiation in HNSCC. Nuclear ING5 may modulate the transactivation of target genes, and may promote apoptosis and cell cycle arrest by interacting with the p300 and p21 proteins. ING5 may function as a tumor suppressor gene or oncogene tightly linked with p53 status, and may play an important role in the prognosis of HNSCC patients. Therefore, we propose that ING5 represents a novel potential molecular therapeutic target for HNSCC.